Great strides have recently been made in the development of nontoxic transpulmonary ultrasonic contrast agents and time domain blood flowmetry. The proposed research is a marriage of these two new developments. Time domain blood flow estimation although has been proven to be feasible and to have several advantages over ultrasonic Doppler methods suffers from low signal to noise ratio because the signal backscattered from blood is extremely weak, By using ultrasonic contrast agents, this problem can be circumvented. Preliminary results have demonstrated the potential of ultrasonic contrast blood flowmetry. In this research, the following approaches on time domain blood flowmetry will be pursued in vitro in conjunction with the utilization of ultrasonic contrast media in a mock flow loop. (1) Mean blood flow velocity will be estimated from a measurement of the time course of the variation in the ultrasonic attenuation of blood following the bolus injection of a contrast agent based upon the indicator dilution principle. (2) Mean velocity and velocity profile in a vessel will be determined using an optical correlator from recorded video ultrasonic images following bolus injection of a contrast agent. (3) Flow field in 2-D will be determined using a two-probe arrangement from cross-correlation of the RF signals detected by these two probes. The proposed work is exploratory in nature and intends primarily to show the feasibility of these approaches in a fundamental way. Once the feasibility of an approach is established, research in a larger scale involving animals will ensue.